Molecular Networks Created by Charge-Assisted Hydrogen Bonding

Article pubs.acs.org/crystal

Molecular Networks Created by Charge-Assisted Hydrogen Bonding in Phosphonate, Phosphate, and Sulfonate Salts of Bis(amidines) Sharon Lie, Thierry Maris, and James D. Wuest* Département de Chimie, Université de Montréal, Montréal, Québec H3C 3J7 Canada S Supporting Information *

ABSTRACT: Two bis(amidines), 2,2′-bi-2-imidazoline (BI) and fluoflavin (FF), were treated with phosphonic, phosphoric, and sulfonic acids in an effort to produce crystalline salts composed of ions linked by networks of charge-assisted hydrogen bonds. As intended, mixing bis(amidine) BI with 1,4-benzenediphosphonic acid and 1,3,5-benzenetriphosphonic acid yielded crystals of phosphonate salts of dication H2BI+2. Structural analyses showed that such salts tend to incorporate tapes composed of alternating dications and anions linked by multiple charge-assisted N−H···O hydrogen bonds of type R22(9) and R12(7). Typically, the ionic tapes are further connected to form sheets or other assemblies by additional O−H···O hydrogen bonds involving phosphonate anions. An analogous reaction of the more weakly basic bis(amidine) FF with 1,4-benzenedisulfonic acid yielded only a sulfonate salt of monocation HFF+; however, diprotonation could be achieved by phosphoric acid to produce a crystalline salt built from stacks of H2FF+2 dications linked to phosphate anions by charge-assisted N−H···O hydrogen bonds of type R22(8). Together, these results demonstrate that acids with multiple PO(OH)2 and SO2OH groups can react with bis(amidines) to produce salts with structural features resulting predictably from the geometry of the ions and their ability to engage in multiple charge-assisted hydrogen bonds according to standard patterns.

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INTRODUCTION Molecular organization in materials reflects the topology of the individual components and the combined effects of diverse intermolecular interactions. When the structures are welldefined, when particular interactions are dominant, and when these factors work in synergy to control how neighboring molecules are positioned, overall organization can often be predicted with confidence.1,2 Among interactions most suitable for controlling organization in this way are hydrogen bonds, which offer high directionality and strength, particularly when reinforced by electrostatic attraction between species of opposite charge.3 In recent work,4 we have shown that such charge-assisted hydrogen bonding can be used to control molecular organization in crystals of carboxylate salts derived from 2,2′bi-2-imidazoline (BI),5 a bis(amidine). Dicarboxylic acids strong enough to convert bis(amidine) BI into dication H2BI+2 by double protonation were found to yield structures built from tapes of alternating dicarboxylate dianions and bis(amidinium) dications, linked by cyclic arrays of chargeassisted hydrogen bonds (Figure 1a). These arrays can be described by the graph set R22(9), where R denotes a cyclic motif, the superscript and subscript give the number of hydrogen-bond acceptors and donors, respectively, and the size of the ring is indicated in parentheses.6 Adjacent tapes pack closely to form sheets, which then stack to form the observed three-dimensional structures. In contrast, weaker dicarboxylic acids produce only monocation HBI+ and favor a second family © 2014 American Chemical Society

Figure 1. Representation of ionic tapes linked by charge-assisted hydrogen bonds according to the graph set R22(9), formed by the interaction of dicarboxylate dianions with (a) bis(amidinium) dication H2BI+2 and (b) the hydrogen-bonded dimer of monocation HBI+.

of tapes (Figure 1b), in which hydrogen-bonded dimers of the monocation are connected to dicarboxylate dianions by chargeassisted hydrogen bonds of type R22(9). For example, oxalic acid (pKa1 1.23) effects double protonation of bis(amidine) BI, and the resulting salt crystallizes according to the motif shown in Figure 1a. However, weaker dicarboxylic acids such as fumaric acid (pKa1 3.02) and terephthalic acid (pKa1 3.51) only monoprotonate the bis(amidine), and their salts have structures of the type shown in Figure 1b. As a result, efforts to engineer Received: May 19, 2014 Published: May 20, 2014 3658

dx.doi.org/10.1021/cg500726v | Cryst. Growth Des. 2014, 14, 3658−3666

Crystal Growth & Design

Article

expectations, including double protonation of bis(amidine) BI and the formation of tapes built from an alternating arrangement of bis(amidinium) dications (H2BI+2) and benzenediphosphonate dianions (H2BDP−2), interconnected by charge-assisted N−H···O hydrogen bonds (Figure 2). In addition, each dianion forms O−H···O hydrogen bonds to two other dianions in adjacent tapes, thus leading to the formation of slightly corrugated sheets, which stack to form the observed structure (Figure 2). Detailed examination of the structure suggests that the bis(amidinium) dications and diphosphonate dianions are linked by seven-membered rings incorporating two chargeassisted N−H···O hydrogen bonds according to motif R12(7). Related motifs have been reported previously.15 Full transfer of two protons from the diphosphonic acid to the bis(amidine) is consistent with the observation of two nearly equal C−N bond lengths within each R12(7) motif (1.306(3) and 1.308(3) Å). In this motif, a single anionic oxygen atom of each phosphonate moiety serves as a bifurcated acceptor of two hydrogen bonds. In contrast, cations derived from bis(amidine) BI have been shown to interact with analogous carboxylate anions by forming nine-membered rings incorporating charge-assisted N−H···O hydrogen bonds of alternative type R22(9).4 This difference in behavior can be attributed in large part to geometric factors, including the length of P−O bonds in phosphonates (which are longer than C−O bonds in carboxylates) and the size of O−P− O angles (which are smaller than O−C−O angles in carboxylates). Specifically, the lengths of the P−O− bonds in phosphonate salt (H2BI+2) (H2BDP−2) are 1.512(2) and 1.510(2) Å, and the O−P−O angle is 116.0(1)°; in contrast, the average length of the corresponding C−O bonds in the oxalate salt is 1.257(1) Å, and the average O−C−O angle is 125.8(2)°. Despite the preference of phosphonate salt (H2BI+2)(H2BDP−2) for the R12(7) motif, however, metalcoordinated derivatives of bis(amidine) BI and related compounds have been observed to engage in hydrogen bonds of type R22(9) with phosphate anions, at least when N−C−C angles within the motif are opened by the effect of coordination.16,17 In bis(amidinium) phosphonate (H2BI+2)(H2BDP−2), N− H···O hydrogen bonds that link the dications and dianions into tapes have relatively short N···O distances (2.706(2) and 2.850(3) Å), and the N−H···O bond angles have an average value of 165.0°. In the analogous oxalate salt, the N···O distance is slightly shorter (2.656 Å), presumably because hydrogen bonds to the acceptor are not bifurcated, and the N− H···O bond angle is 160.7°.18 Both salts have N···O distances shorter than those normally observed in nonionic cocrystals incorporating N···H−O interactions without full protonation of N (2.85 Å).19,20 We conclude that the hydrogen bonds in phosphonate salt (H2BI+2)(H2BDP−2) are strengthened by charge assistance. Adjacent ionic tapes in the phosphonate salt are linked to form sheets by O−H···O hydrogen bonding characteristic of simple arylphosphonic acids and their monoanions (Figure 2a), which form chair-shaped pairs defined by cyclic motif R22(8).15,21−24 The O···O distance (2.595(2) Å) and O−H··· O angle (173°) are normal. As a result, all ions in each sheet are interconnected by a two-dimensional network of hydrogen bonds; in contrast, hydrogen bonding in sheets of the corresponding oxalate is confined to one dimension, aligned with individual tapes. Sheets in the phosphonate salt stack to

crystals derived from bis(amidine) BI and compounds with multiple COOH groups must take values of pKa into account.

To avoid this complication, we turned to salts prepared by treating bis(amidines) with phosphonic acids and sulfonic acids, which are much stronger than the corresponding carboxylic acids.7−9 We reasoned that compounds with multiple PO(OH)2 and SO2OH groups would reliably diprotonate bis(amidine) BI and close analogues, thereby producing a single family of networks in which bis(amidinium) dications are linked to phosphonate and sulfonate anions by multiple chargeassisted hydrogen bonds. To test this possibility, we have made salts of two related bis(amidines), compound BI and fluoflavin (FF), with various complex phosphonic and sulfonic acids, including 1,4-benzenediphosphonic acid (H4BDP), 1,3,5benzenetriphosphonic acid (H6BTP), and 1,4-benzenedisulfonic acid (H2BDS). After significant effort, we crystallized several of these salts and simple analogues, and we now report their structures.

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RESULTS AND DISCUSSION BI and FF10 were prepared by known methods and mixed in specific ratios with phosphonic, phosphoric, and sulfonic acids, which were obtained as described in the Experimental Section. Both bis(amidines) BI and FF reacted with the various acids to give salts, due to the large differences in pKa between those of typical amidinium cations (10−11)11−14 and those of arylphosphonic and arylsulfonic acids.7−9 Single crystals of salts were grown from various solvents, and their structures were resolved by X-ray diffraction to confirm the composition of the salts and to reveal the nature of the interionic interactions. 2,2′-Bi-2-imidazolinium 1,4-benzenediphosphonate (H2BI+2)(H2BDP−2). Crystals were obtained by preparing separate solutions of bis(amidine) BI and H4BDP in 3:2 EtOH/H2O, mixing them in a 1:1 molar ratio, and allowing the combined solution to undergo slow evaporation at 25 °C. This procedure yielded crystals of two forms, one with the composition (H2BI+2)(H2BDP−2). Crystals of this form could also be grown from DMSO. Their structure was determined by X-ray diffraction, and crystallographic parameters are summarized in Table 1. Our structural analyses were consistent with 5

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Table 1. Crystallographic Data for Salts (H2BI+2)(H2BDP−2), (H2BI+2)(H2BDP−2)·2DMSO, (H2BI+2)(H2BDP−2)·4H2O, (H2BI+2)(H4BTP−2)·DMSO, (HFF+)2(BDS−2)·4DMSO, and (H2FF+2)[PO2(OH)2−]2·H2O salt crystallization medium formula crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Z′ T (K) ρcalc (g cm−3) μ (mm−1) crystal size (mm) θ range (deg) measured reflections independent reflections R1, I > 2σ(I) R1, all data wR2, I > 2σ(I) wR2, all data GOF

(H2BI+2)(H2BDP−2)

(H2BI+2)(H2BDP−2)· 2DMSO

(H2BI+2)(H2BDP−2)· 4H2O

(H2BI+2)(H4BTP−2)· DMSO

(HFF+)2(BDS−2)· 4DMSO

(H2FF+2) [PO2(OH)2−]2·H2O

DMSO

DMSO

EtOH/H2O

DMSO

DMSO

heptanoic acid

C12H18N4O6P2 monoclinic P21/n 9.0366(5) 8.0286(4) 10.8628(6) 90 107.809(3) 90 750.34(7) 2 0.5 100 1.665 3.030 0.08 × 0.07 × 0.04 5.59−70.95 25 205

C16H30N4O8P2S2 triclinic P1̅ 4.8022(5) 10.3715(10) 12.1080(11) 99.771(5) 93.061(6) 99.947(5) 583.24(10) 1 0.5 150 1.516 3.820 0.08 × 0.08 × 0.04 3.72−68.05 10 321

C12H26N4O10P2 monoclinic P21/c 10.1046(5) 8.5162(5) 11.6426(6) 90 109.58(3) 90 943.9(2) 2 0.5 100 1.577 2.667 0.15 × 0.11 × 0.10 4.64−70.82 6920

C14H25N4O10P3S triclinic P1̅ 7.513(2) 11.899(3) 12.077(3) 95.688(13) 94.235(14) 94.317(13) 1067.8(5) 2 1 150 1.662 4.049 0.18 × 0.11 × 0.06 3.69−69.72 18 830

C42H50N8O10S6 triclinic P1̅ 8.4966(4) 10.1445(5) 15.0176(7) 74.608(2) 81.509(1) 66.496(2) 1143.17(9) 1 0.5 150 1.481 3.329 0.12 × 0.12 × 0.10 3.06−69.98 19 045

C14H18N4O9P2 monoclinic C2/c 22.7176(15) 4.5011(3) 20.409(2) 90 122.834(2) 90 1753.5(2) 4 0.5 150 1.698 2.842 0.11 × 0.07 × 0.05 4.63−68.48 37 239

1442

2109

1784

3458

4232

1613

0.0474 0.0519 0.1283 0.1327 1.065

0.0796 0.0885 0.2157 0.2222 1.097

0.0435 0.0507 0.1145 0.1213 1.040

0.1244 0.1574 0.3369 0.3599 1.290

0.0360 0.0364 0.1001 0.1006 1.025

0.0616 0.0635 0.1350 0.1362 1.234

R12(7). Appearance of the ionic tapes in both structures suggests that they are a dominant associative motif. 2,2′-Bi-2-imidazolinium 1,4-Benzenediphosphonate (H2BI+2)(H2BDP−2)·4H2O. In addition to producing unsolvated crystals of composition (H2BI+2)(H2BDP−2), mixing bis(amidine) BI and 1,4-benzenediphosphonic acid (H4BPD) in a 1:1 ratio in 3:2 EtOH/H2O also produced crystals of tetrahydrate (H2BI+2)(H2BDP−2)·4H2O. The structure of this additional form was resolved, and Table 1 summarizes various crystallographic parameters. Again, the data suggest that complete transfer of two protons from the diphosphonic acid to the bis(amidine) was achieved, and the resulting diphosphonate dianions and bis(amidinium) dications participate in the formation of an extensively hydrogen-bonded network. However, the dianions and dications are not linked directly to form normal tapes; instead, they interact indirectly through intervening molecules of H2O (Figure 4). Detailed analysis of the structure revealed chains of 1,4benzenediphosphonate dianions linked end-to-end along the b axis by normal O−H···O hydrogen bonding of type R22(8), as shown in Figure 4a. The resulting hydrogen-bonded rings adopt chair conformations with normal O···O distances (2.598(2) Å) and O−H···O angles (166°). Each diphosphonate dianion forms additional hydrogen bonds with six molecules of H2O, some of which serve to connect adjacent chains to form corrugated anionic sheets parallel to the bc plane. Each bis(amidinium) dication interacts with two molecules of H2O to form R12(7) motifs in which the atoms of oxygen act as acceptors of bifurcated N−H···O hydrogen bonds (Figure 4b), with normal N···O distances (2.773(3) and 2.786(3) Å) and

give the observed structure, with no noteworthy interactions between the sheets (Figure 2b). 2,2′-Bi-2-imidazolinium 1,4-Benzenediphosphonate (H2BI+2)(H2BDP−2)·2DMSO. In addition to producing crystals of composition (H2BI+2)(H2BDP−2), mixing bis(amidine) BI and 1,4-benzenediphosphonic acid (H4BPD) in a 1:1 ratio in DMSO also yielded crystals of composition (H2BI+2)(H2BDP−2)·2DMSO. The structure of this second form was resolved, and crystallographic parameters are presented in Table 1. Analysis again suggested the full transfer of two protons from the diphosphonic acid to the bis(amidine) and revealed the expected presence of tapes built from alternating bis(amidinium) dications and diphosphonate dianions (Figure 3a), connected by charge-assisted N−H···O hydrogen bonds of type R12(7). In these motifs, the N···O distances (2.669(5) and 2.757(5) Å) and average N−H···O angle (163°) have values that are normal for charge-assisted hydrogen bonds. The tapes lie along the ab diagonal and are connected to form sheets in the ab plane by O−H···O hydrogen bonding of phosphonate anions (Figure 3a), with normal O···O distances (2.552(5) Å) and O−H···O angles (167°). These interactions create columns of diphosphonate dianions stacked within the sheets along the a axis. In the observed structure, the ionic sheets are separated by intervening layers of DMSO (Figure 3b). We conclude that crystals of the salt (H2BI+2)(H2BDP−2), both as the DMSO solvate and the unsolvated form, incorporate closely similar tapes made up of alternating bis(amidinium) dications and diphosphonate dianions held together by charge-assisted N−H···O hydrogen bonds of type 3660



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Figure 3. Representations of the structure of crystals of 2,2′-bi-2imidazolinium 1,4-benzenediphosphonate (H2BI+2) (H2BDP−2)· 2DMSO grown from DMSO. (a) View of part of three tapes composed of alternating bis(amidinium) dications (H2BI+2) and diphosphonate dianions (H2BDP−2), linked by charge-assisted N−H··· O hydrogen bonds of type R12(7). The ionic tapes are interconnected to form sheets by additional O−H···O hydrogen bonding of phosphonate anions. Hydrogen bonds are shown as broken lines. (b) Side view showing how the ionic sheets are separated by layers of included DMSO. In both images, atoms of carbon are shown in gray, hydrogen in white, nitrogen in blue, oxygen in red, phosphorus in purple, and sulfur in yellow.

Figure 2. Representations of the structure of crystals of 2,2′-bi-2imidazolinium 1,4-benzenediphosphonate (H2BI+2) (H2BDP−2) grown from DMSO or EtOH/H2O. (a) View of part of three tapes composed of alternating bis(amidinium) dications (H2BI+2) and diphosphonate dianions (H2BDP−2), linked by charge-assisted N−H··· O hydrogen bonds of type R12(7). The ionic tapes are interconnected to form slightly corrugated sheets by O−H···O hydrogen bonding of phosphonate anions according to standard motif R22(8). Hydrogen bonds are shown as broken lines. (b) Side view showing stacking of the sheets. In both images, atoms of carbon are shown in gray, hydrogen in white, nitrogen in blue, oxygen in red, and phosphorus in purple.

acceptor to produce a standard bifurcated R12(7) motif, as seen in other amidinium phosphonates derived from compound BI. In this second motif, the N···O distances are 2.66(1) and 2.77(11) Å, and the corresponding N−H···O angles are 163 and 159°, respectively. It is noteworthy that the tapes contain two types of bis(amidinium) dications, one linked to phosphonate anions only by R12(7) motifs and the other linked only by R22(9) motifs. Of the four N−H···O hydrogen bonds formed by each bridging phosphonate, only those involving the neutral OH group as acceptor have N···O distances longer than 2.85 Å, suggesting that the other hydrogen bonds are strengthened by charge assistance. The increased N···O(H) distance and the corresponding bent N−H···O(H) angle presumably facilitate formation of the R22(9) motif, which appears to be disfavored in other phosphonate salts of dication H2BI2+ for geometric reasons. The tapes are linked by additional O−H···O hydrogen bonds between 1,3,5-benzenetriphosphonate dianions. This creates conspicuous anionic stacks aligned with the a axis and held together by a total of eight O−H···O hydrogen bonds between each dianion and its two neighbors (Figure 5b). The O···O distances (2.53(1), 2.55(1), 2.60(1), and 2.66(1) Å) are normal, and most of the O−H···O angles (176, 171, 139, and 176°, respectively) are nearly linear. Large distances between the centroids of adjacent benzenetriphosphonate units within each stack (3.72(1) and 3.82(1) Å) provide no evidence of important stabilizing aromatic interactions. The anionic stacks

average N−H···O angle (166(3)°). Each dihydrated dication is then linked by hydrogen bonds involving H2O to six surrounding diphosphonate dianions. The overall structure can be described as being composed of corrugated anionic hydrogen-bonded sheets of diphosphonate dianions, separated by layers of bis(amidinium) dications (Figure 4c). 2,2′-Bi-2-imidazolinium 1,3,5-Benzenetriphosphonate (H2BI+2)(H4BTP−2)·DMSO. Crystals of the salt (H2BI+2)(H4BTP−2)·DMSO were obtained by mixing bis(amidine) BI and 1,3,5-benzenetriphosphonic acid (H6BTP) in a 3:2 molar ratio in DMSO and by allowing the solution to stand at 25 °C. The structure was determined, and crystallographic parameters are summarized in Table 1. Analysis suggested that dication H2BI+2 was formed as expected by the transfer of two protons from triphosphonic acid H6BTP to bis(amidine) BI. The structure incorporates parallel tapes built from an alternating arrangement of bis(amidinium) dications and triphosphonate dianions linked by charge-assisted N−H···O hydrogen bonds (Figure 5a). In the tapes, each dication is connected to the next by a single bridging phosphonate group, which engages in two distinct cyclic patterns of hydrogen bonding. One gives a ninemembered motif R22(9), in which both O− and OH of the phosphonate group serve as acceptors. In this motif, the N···O distances are 2.69(1) and 2.98(1) Å, and the corresponding N− H···O angles are 165 and 142°, respectively. In the second pattern of charge-assisted hydrogen bonding, a single O− acts as 3661



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Figure 4. Representations of the structure of crystals of 2,2′-bi-2-imidazolinium 1,4-benzenediphosphonate (H2BI+2)(H2BDP−2)·4H2O grown from 3:2 EtOH/H2O. (a) View of part of a hydrogen-bonded chain of 1,4-benzenediphosphonate dianions and associated molecules of H2O. (b) View of a dihydrated bis(amidinium) dication and surrounding diphosphonate dianions, linked indirectly by hydrogen bonds to intervening molecules of H2O. (c) Side view of corrugated anionic sheets of diphosphonate dianions and intervening layers of bis(amidinium) dications. Atoms of carbon are shown in gray, hydrogen in white, nitrogen in blue, oxygen in red, and phosphorus in purple. Hydrogen bonds are shown as broken lines.

molar ratio in hot DMSO, and the mixture was cooled to produce crystals of composition (HFF+)2(BDS−2)·4DMSO. Their structure was determined, and crystallographic parameters are provided in Table 1. Bis(amidine) BI is diprotonated by acids with values of pKa below about 1, but FF is more weakly basic and is only monoprotonated by disulfonic acid H2BDS in the observed structure. HFF+ monocations are linked to bridging disulfonate dianions by charge-assisted N−H···O hydrogen bonds of type R22(8), as shown in Figure 6a. Evidence for protonated fluoflavin HFF+ is provided by the nearly identical lengths of the two C−N bonds incorporated in the R22(8) motif (1.329(2) and 1.330(2) Å), whereas the lengths of the two other C−N bonds in the monocation are distinctly different (1.303(2) and 1.355(2) Å). N···O distances (2.735(2) and 2.816(2) Å) and N−H···O angles (174°) in the salt have values that are characteristic of charge-assisted hydrogen bonds and do not support an earlier suggestion that sulfonates tend to form longer hydrogen bonds than carboxylates or phosphonates.18 It is noteworthy that monocation HFF+ associates with sulfonate to form an R22(8) motif incorporating a tetrahedral O−S−O angle (111.36(6)°), whereas analogue HBI+ prefers bifurcated R12(7) motifs in its phosphonate salts, rather than alternative R22(9) patterns with tetrahedral O−P−O angles. The behavior of cations HFF+ and HBI+ is presumably based in both cases on a preference for N−H···O angles that are nearly linear. Carboxylate-bridged monocations HBI+ are typically further linked to form tapes by N−H···N hydrogen bonds of type R22(10), as shown in Figure 1b. In contrast, similar extended association is not observed in fluoflavinium salt (HFF+)2(BDS−2), presumably because it would create repulsive

are closely analogous to neutral hydrogen-bonded columns observed in the structure of 1,3,5-benzenetriphosphonic acid itself.25 The overall structure of the salt (H2BI+2)(H4BTP−2)· DMSO can be described as consisting of parallel tapes of alternating hydrogen-bonded bis(amidinium) dications and triphosphonate dianions, which pack in a way that allows the dianions to form hydrogen-bonded stacks, with molecules of DMSO included in remaining spaces (Figure 5c−e). Fluoflavinium 1,4-Benzenedisulfonate (HFF+)2(BDS−2)· 4DMSO. Our observations suggest that protonation of bis(amidine) BI by acids with multiple COOH or PO(OH)2 groups produces salts that crystallize to form structures in which networks of charge-assisted hydrogen bonds help position the ionic components according to established patterns. To explore the scope of this strategy, we replaced compound BI with FF, a bis(amidine) closely related to tetracene. Long acenes promise to be useful as components in molecular optoelectronic devices;26 unfortunately, however, simple hydrocarbons of this type are poorly soluble, highly reactive, and unable to engage in intermolecular interactions that can readily be used to alter properties by controlling how neighboring molecules are positioned. FF and related heterocyclic compounds are attractive alternatives because substituting CH in simple acenes by N or NH creates opportunities to solubilize by forming salts, adjust bandgaps, and control molecular organization by forming hydrogen bonds, including those assisted by charge.27,28 To assess this potential, we treated FF with various complex carboxylic, phosphonic, and sulfonic acids, but few attempts to grow crystals were successful. In one, FF and 1,4benzenedisulfonic acid (H2BDS) were combined in a 1:1 3662



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Figure 5. Representations of the structure of crystals of 2,2′-bi-2-imidazolinium 1,3,5-benzenetriphosphonate (H2BI+2)(H4BTP−2)·DMSO grown from DMSO. (a) View of part of a tape built from alternating bis(amidinium) dications and triphosphonate dianions linked by charge-assisted N− H···O hydrogen bonds of types R22(9) and R12(7). (b) View of part of a hydrogen-bonded stack of triphosphonate dianions. (c) View showing how adjacent tapes pack to create hydrogen-bonded stacks of triphosphonate dianions. (d) View along the a axis showing how the anionic stacks are separated by intervening bis(amidinium) dications and included molecules of DMSO. (e) Similar view with molecules of DMSO omitted. Atoms of carbon are shown in gray, hydrogen in white, nitrogen in blue, oxygen in red, phosphorus in purple, and sulfur in yellow. Hydrogen bonds are shown as broken lines.

was heated, and subsequent cooling produced crystals of a phosphate salt of composition (H2FF+2)[PO2(OH)2−]2·H2O. Phosphate is presumably introduced as a contaminant in pyrophosphate by partial hydrolysis. The structure of the salt was resolved, and Table 1 summarizes the crystallographic data. As planned, the salt incorporates H2FF+2 dications linked to two monophosphate monoanions (in part statistically disordered over two positions) by a total of four N−H···O hydrogen bonds of type R22(8), as shown in Figure 8a. Formation of diprotonated fluoflavin (H2FF+2) is consistent with the observation of similar C−N distances within the R22(8) motifs

H···H interactions involving adjacent C−H bonds (Figure 7). Instead, HFF+ monocations form additional hydrogen bonds with included molecules of DMSO (Figure 6a). The resulting hydrogen-bonded (HFF+)2(BDS−2) units pack to form a structure with alternating layers of disulfonate dianions and HFF+ monocations stacked in offset with an interplanar separation of approximately 3.8 Å (Figure 6b). Fluoflavinium Monophosphate (H2FF+2)[PO2(OH)2−]2· H2O. The failure of a sulfonic acid to diprotonate FF prompted us to examine stronger acids. FF and pyrophosphoric acid were combined in a 1:1 molar ratio in heptanoic acid, the mixture 3663



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Figure 6. Representations of the structure of crystals of fluoflavinium 1,4-benzenedisulfonate (HFF + ) 2 (BDS −2 )·4DMSO grown from DMSO. (a) View showing fluoflavinium monocations linked to a bridging disulfonate dianion by charge-assisted N−H···O hydrogen bonds of type R22(8) and to two included molecules of DMSO by simple N−H···O hydrogen bonds. Atoms of carbon are shown in gray, hydrogen in white, nitrogen in blue, oxygen in red, and sulfur in yellow. Hydrogen bonds are shown as broken lines. (b) View showing how the hydrogen-bonded (HFF+)2(BDS−2) units pack to create offset stacks of fluoflavinium monocations. Cations appear in blue and dianions in red, with guest molecules of DMSO omitted for clarity.

Figure 8. Representations of the structure of crystals of fluoflavinium monophosphate (H2FF+2)[PO2(OH)2−]2·H2O grown from heptanoic acid. (a) View showing one of two nonequivalent ionic columns composed of stacked H2FF+2 dications and monophosphate anions. Dications are linked to monoanions by charge-assisted N−H···O hydrogen bonds of type R22(8), and monoanions are further connected by O−H···O hydrogen bonds. (b) View showing how adjacent columns are linked by additional O−H···O hydrogen bonds involving monophosphate anions and included molecules of H2O. (c) View showing alternating ionic layers of monophosphate anions (red) and H2FF+2 dications (blue) packed in a herringbone motif, with guest molecules of H2O omitted for clarity. Unless otherwise indicated, atoms of carbon are shown in gray, hydrogen in white, nitrogen in blue, oxygen in red, and phosphorus in purple. Hydrogen bonds are shown as broken lines.

Figure 7. Representation of hypothetical repulsive H···H interactions arising when pairs of fluoflavinium monocations are linked by N−H··· N hydrogen bonds of type R22(8).

(1.321(5)−1.336(5) Å), and the N···O distances (2.641(6)− 2.749(6) Å) and N−H···O angles (160−172°) are normal. Additional single O−H···O hydrogen bonds with normal O···O distances (2.701(7) Å) link the monophosphate anions to create ionic columns in which H2FF+2 dications stack in offset with an interplanar separation of approximately 3.5 Å (Figure 8a). Monophosphate anions in adjacent columns are further linked by additional O−H···O hydrogen bonds, some involving included molecules of H2O (Figure 8b). The overall structure consists of alternating ionic layers, one composed of extensively hydrogen-bonded monophosphate anions and the other containing H2FF+2 dications packed in a herringbone pattern (Figure 8c).

produce salts of monocation HBI+. In contrast, phosphonic acids and sulfonic acids are strong enough to reliably produce salts of dication H2BI+2. As intended, mixing bis(amidine) BI with 1,4-benzenediphosphonic acid and 1,3,5-benzenetriphosphonic acid yielded crystals of phosphonate salts of dication H2BI+2. Structural analyses showed that such salts tend to incorporate tapes composed of alternating dications and anions linked by multiple charge-assisted N−H···O hydrogen bonds of type R22(9) and R12(7). Typically, the ionic tapes are further connected to form sheets or other assemblies by additional O− H···O hydrogen bonding of phosphonate anions. FF, a more weakly basic analogue of bis(amidine) BI, reacted with 1,4benzenedisulfonic acid to produce crystals of a salt of monoprotonated cation HFF+. However, diprotonation could

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CONCLUSIONS 2,2′-Bi-2-imidazoline (BI) is known to react with compounds containing multiple COOH groups to give carboxylate salts that crystallize as ionic networks held together in part by characteristic charge-assisted N−H···O hydrogen bonds of type R22(9). Typical carboxylic acids are only acidic enough to 3664



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parameters were performed with the APEX2 suite of software, final lattice parameters and integrated intensities were calculated using SAINT software,33 and a multiscan absorption correction was applied using SADABS.34 Structures were solved with SHELXS-97 and refined with SHELXL-97.35 All non-hydrogen atoms were refined with anisotropic thermal displacement parameters. Hydrogen atoms were first located from the calculated difference-Fourier maps and then refined as riding atoms using the standard models from SHELXL-97. In all structures with included DMSO, the guest molecules were found to be disordered. Also disordered were included H2O and PO2(OH)2− in the salt (H2FF+2)[PO2(OH)2−]2·H2O, the H2BDP−2 dianion in the salt (H2BI+2)(H2BDP−2)·2DMSO, and one phosphonate group in the salt (H2BI+2)(H4BTP−2)·DMSO. In most of the structures, the cations (except in the case of (HFF+)2(BDS−2)·(4DMSO) and anions (with the exception of (H2 BI+2 )(H4 BTP −2 )·DMSO and (H 2 FF +2 )[PO2(OH)2−]2·H2O) lie on special positions, leading to structures with Z′ = 0.5. Structural analysis of the salt (H2BI+2)(H4BTP−2)· DMSO is of lower quality than the others because the crystals were twinned and decomposed, leading to truncation of the collection of final data. When sufficient amounts of crystalline material were available, assessments of homogeneity were carried out by comparing calculated diffraction patterns (generated using Mercury software)36 with experimental patterns measured on a Bruker D8 Discover diffractometer with copper radiation (CuKα, λ = 1.5418 Å). For the three samples analyzed in this way, calculated and experimental patterns were closely similar.37

be achieved by treatment with phosphoric acid, which gave crystals incorporating H2FF+2 dications linked to phosphate anions by charge-assisted N−H···O hydrogen bonds of type R22(8). These observations, along with recent related studies,4 demonstrate that salts produced by reactions of bis(amidines) with complex carboxylic, phosphonic, and sulfonic acids yield crystals with structural features resulting predictably from the geometry of the ions and their ability to engage in multiple charge-assisted hydrogen bonds according to standard patterns.

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EXPERIMENTAL SECTION

General Notes. BI,5 FF,10 and H2BDS29 were prepared by published methods. Ethyl esters of diphosphonic acid H4BDP and triphosphonic acid H6BTP were made by reported procedures30 and converted into the corresponding acids by hydrolysis under standard conditions.31,32 Other compounds were purchased from commercial sources and used without further purification. Syntheses and Characterizations of Salts. Separate solutions of BI and various phosphonic acids were prepared in 3:2 EtOH/H2O or DMSO. The solutions were filtered, mixed in various molar ratios (1:1 for H4BDP or 3:2 for H6BTP), and kept at 25 °C to allow crystallization to occur. FF and either disulfonic acid H2BDS or pyrophosphoric acid were combined in a 1:1 ratio, a small volume of solvent was added (DMSO for H2BDS or heptanoic acid for pyrophosphoric acid), and the resulting mixture was stirred and heated briefly until the solids had dissolved. Crystallization occurred while the resulting solution was kept at 25 °C. 2,2′-Bi-2-imidazolinium 1,4-Benzenediphosphonate (H2BI+2)(H2BDP−2). Isolated in 6% yield as colorless air-stable crystals: 1 H NMR (700 MHz, D2O) δ 3.95 (s, 8H), 7.64 (m, 4H); 13C NMR (175 MHz, D2O) δ 45.9, 129.9, 137.2 (d, 1JCP = 174 Hz), 152.5; 31P NMR (283 MHz, D2O) δ 12.4; HRMS (+ESI) calcd. for C6H12N4+2 m/e 70.05310, found 70.02919; HRMS (-ESI) calcd. for C6H7O6P2− m/e 236.97179, found 236.97653. 2,2′-Bi-2-imidazolinium 1,4-Benzenediphosphonate (H2BI+2)(H2BDP−2)·2DMSO. Isolated along with the unsolvated form as colorless air-stable crystals. 2,2′-Bi-2-imidazolinium 1,4-Benzenediphosphonate (H2BI+2)(H2BDP−2)·4H2O. Isolated in 47% yield as colorless air-stable crystals: mp > 250 °C (dec); FTIR (ATR) 3300−2500 (br), 1598, 1576, 1290, 1127, 1058, 897 cm−1; 1H NMR (700 MHz, D2O) δ 3.91 (s, 8H), 7.63 (m, 4H); 31P NMR (283 MHz, D2O) δ 12.32; HRMS (+ESI) calcd. for C6H12N4+2 m/e 70.05310, found 70.02919; HRMS (-ESI) calcd. for C6H7O6P2− m/e 236.97179, found 236.97222. Anal. Calcd for C12H26N4O10P2: C, 32.13; H, 5.85; N, 12.50. Found: C, 32.82; H, 5.57; N, 12.49. 2,2′-Bi-2-imidazolinium 1,3,5-Benzenetriphosphonate (H2BI+2)(H4BTP−2)·DMSO. Isolated in 34% yield as colorless airstable crystals: mp > 200 °C (dec); FTIR (ATR) 3200−2100 (br), 1600, 1572, 1289, 1131, 938 cm−1; 1H NMR (700 MHz, D2O) δ 2.55 (s, 6H), 3.89 (s, 8H), 8.00 (s, 3H); 31P NMR (283 MHz, D2O) δ 12.2; HRMS (+ESI) calcd. for C6H12N4+2 m/e 70.05310, found 70.02980; HRMS (-ESI) calcd. for C6H8O9P3− m/e 316.93812, found 316.94431. Fluoflavinium 1,4-Benzenedisulfonate (HFF + ) 2 (BDS −2 )· 4DMSO. FTIR (ATR) 3000 (br), 1694, 1556, 1486, 1232, 1126, 1023, 997 cm−1; HRMS (-ESI) calcd. for C6H6O6S2−2 m/e 118.98029, found 118.97143. Anal. Calcd for C34H26N8O6S2 · 4DMSO: C, 49.50; H, 4.95; N, 11.00; S, 18.84. Found: C, 48.84; H, 4.80; N, 10.86; S, 18.51. Fluoflavinium Monophosphate (H2FF+2)[PO2(OH)2−]2·H2O. Isolated in amounts insufficient for extensive characterization. X-ray Crystallography. Structural analyses of single crystals were carried out in one of two ways: (1) At 150 K with a Bruker AXS X8/ Proteum Microstar diffractometer, using Cu Kα radiation produced by an FR591 rotating-anode generator equipped with multilayer HELIOS optics (all salts except for (H2BI+2)(H2BDP−2) and (H2BI+2)(H2BDP−2)·4H2O); or (2) at 100 K using a Bruker APEX diffractometer equipped with an Incoatec microsource generator. Collections of data and determinations of initial unit-cell lattice

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ASSOCIATED CONTENT

S Supporting Information *

Additional crystallographic details (tables of structural data in CIF format, thermal atomic displacement ellipsoid plots, and comparisons of calculated and experimental powder diffraction patterns). This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Natural Sciences and Engineering Research Council of Canada, the Ministère de l′Éducation du Québec, the Canada Foundation for Innovation, the Canada Research Chairs Program, and Université de Montréal for financial support. In addition, we thank Daniel Beaudoin for helpful advice about phosphonic and sulfonic acids.

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REFERENCES

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Molecular Networks Created by Charge-Assisted Hydrogen Bonding

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